The Battle of the Allia was a battle of the first Gallic invasion of Rome. The battle was fought near the Allia river: the defeat of the Roman army opened the route for the Gauls to sack Rome. It was fought in 390/387 BC.

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Prior to the battle, the Senones, a single tribe of Gaul, traversed the Apennines searching for new land to settle. They eventually camped outside the town of Clusium (in the Etruscan province of Siena) and began negotiations for land rights. The Clusians felt threatened by the Senones, and they called for help from Rome, which had recently exerted military influence over Etruria. Rome, weakened by recent wars, sent a delegation of three ambassadors, the Fabii brothers, to negotiate the situation.[1]

When negotiations broke down, the Clusians sent an army to force the Senones off the land. At this point, the Roman historian Livy stated that the Roman ambassadors "broke the law of nations" (that is, broke their oath of neutrality as ambassadors) "and took up arms" against the Senones. In the resulting action, Quintus Fabius, an ambassador and a member of a powerful patrician family, killed one of the Gallic leaders (a chieftain). When the Senones realized that the sacred trust of the ambassador was broken, they withdrew from battle to discuss the issue.[2]

The Senones sent their own ambassadors to Rome, demanding the Fabians be handed over to them for justice. Many Romans (especially priests) were sympathetic, and agreed that it was a breach of the law of nations. However, the Roman masses mocked the priests, and as Livy writes, "those who ought to have been punished were instead appointed for the coming year military tribunes with consular powers (the highest that could be granted).... The Celtic (Gallic) envoys were naturally - and rightly - indignant!"[3] The enraged Senones promised war against the Romans to avenge the insult that they had been dealt.

The Senones marched 130 km from Clusium to Rome to take revenge. Livy describes their journey:

"Contrary to all expectation the Celts (Gauls) did them [the people of the countryside] no harm, nor took aught from their fields, but even as they passed close by their cities, shouted out that they were marching on Rome and had declared war only on the Romans, but the rest of the people they regarded as friends."[4]

Thus, the Senones came to engage the Roman army about 18 km north of the city, at the Battle of the Allia.

According to the common (but incorrect) Varronian chronology, the battle took place on July 18, 390 BC, but a more plausible date is 387. About 24,000 Romans under Quintus Sulpicius fought against the Senones, a Gallic tribe who were about half that number,[5] under Brennus. The Romans, with six legions, took post on the Allia to check the advance of the Senones on Rome. In those days, a legion counted 4,200 men, but was rarely fully manned. The Roman army was at this time a militia and very similar to a Greek phalanx battle line, with heavy hoplites in the centre (representing the richer Roman citizens) and extending to flanks with poorer and poorly armed conscripts (every soldier was required to supply his own equipment). When the Gauls attacked, the Roman flanks were routed leaving the Roman centre to be surrounded and slaughtered. Many of Rome's older citizens made up this centre and they would be sorely missed in the coming calamity.

While the left wing fled to Veii, the survivors of the right wing fled back to Rome in panic; as Livy states, "all hastened to Rome and took refuge in the Capitol without closing the gates." In Rome the citizens barricaded themselves on the Capitoline Hill. The Gauls tried a full frontal assault, pausing half way up the hill before charging, but they paid dearly and lost many lives. At this point, the besieged Roman soldiers in Veii needed to get a message to the Senate in order to reinstate Marcus Furius Camillus as the dictator and general, and so a messenger climbed a steep cliff the Gauls had neglected to guard. The messenger left with the Senate's approval, but the Gauls noticed this path onto the hill, using it to launch a sneak attack. According to legend Marcus Manlius Capitolinus was alerted to the Gallic attack by the sacred geese of Juno ("the Capitoline Geese"). The rest of the city was plundered and almost all Roman records were destroyed. Marcus Furius Camillus may have arrived with a relief army, but this may be Roman propaganda to help quell the humiliation of defeat. The Gauls may have been ill-prepared for the siege, as an epidemic broke out among them as a result of not burying the dead. Brennus and the Romans negotiated an end to the siege when the Romans agreed to pay one thousand pounds of gold. According to tradition, to add insult to injury, Brennus was discovered using heavier weights than standard for weighing the gold. When the Romans complained, Brennus is said to have thrown his sword and belt on the scales and adding in Latin, "Woe to the vanquished" ("vae victis"), in conclusion (Livy V. 48).

According to some Roman historians, in this very moment Camillus arrived with a Roman army and, after putting his sword on the scale, replied, "Not gold, but steel redeems the native land," thus attacking the Gauls. A battle ensued in the streets of Rome, but neither army could fight effectively in the narrow streets and alleyways. The Gallic and Roman armies left the city and fought the next day. Camillus' army lived up to his hopes and the Gallic army was routed. The Romans dubbed Camillus a "second Romulus," a second founder of Rome.

The Historia Regum Britanniae, a medieval work written c. 1136 by Geoffrey of Monmouth on the legendary kings of Britain, states that Brennius led both Britons and Gauls. He besieged Rome for three days until his brother came to aid in the invasion. The Romans defended the city for many days and were successful in repelling the invaders. Finally, the two consuls put on armour and joined the men defending the city. They pushed the invaders back but Belinus (Brennius' brother) was able to reform the lines and stop the attacks. Brennius and Belinus continued forward until the walls were breached and the Britons and Gauls invaded the city. According to this story Brennius stayed in Rome and ruled ruthlessly for the rest of his days, although this seems highly unlikely.

It was considered that the capital be moved to Veii, but the Senate decided against it when a centurion was overheard saying to his men "Let us stay here."[6]

It is conjectured that there was no effective wall around Rome before the siege because Rome's earlier Etruscan rulers may have forced the Romans to dismantle significant defences. As a result of the siege and near total destruction of Rome, Rome built the much stronger Servian Wall.

The Romans also began restructuring their military organization: They ceased using the Greek phalanx style spear and adopted better and more standardized armour and weapons. The massacre of the 1st Class infantry, representing many Patrician and aristocratic citizens, enforced the need not to expose such important members of Roman society. It could have been at this time that the Triarii (remnants of the 1st Class) were formed, as a reserve.

Many historians speculate that the Romans learned much about weapons technology and battle tactics from this run-in with the Senones. Though only a single tribe, the Senones were part of the much larger culture of Celts (or Gauls) that had more advanced iron-working and close-quarter combat techniques. Specifically, the Celts/Gauls used heavier long swords and full body shields, which allowed them to interlock shields for greater defense (a tactic later named "tortoise" (testudo) in the Roman histories).

Later, after Roman defeats in the second Samnite War had shown novel enemy tactics and formations, the need for increased flexibility was recognized, leading to the reorganization of the legion into three main lines of soldiers: the hastati in front, the principes in the middle, and the triarii in the rear organized in alternating "maniples" (units). This was to be known as "manipular formation". Lightly equipped men who had been fighting in the legion for up to two years would fight in the Velites rank in the far front, throwing javelins at the enemy and then retreating. Men with more experience would fight in the next two ranks armed with a heavy javelin, a short sword and a shield: Hastati in the front, veteran Principes behind them. Finally the older Triarii would be in the rear, organized in smaller units of 60 men as opposed to 120 in the front ranks. The Triarii were armed in Hoplite weapons and armor. The Romans had created a "teaching army" that would introduce the young Velites to battle while minimizing the chances of death.

The military system that resulted remained the basis of all Roman armies for the next few centuries, as well as the instrument that made possible the Roman Empire.

The defeat at the hands of the Gauls was the last time the city of Rome was captured by non-Roman forces until AD 410.